One conventional type of optical fiber has a refractive index profile designed to confine a propagating optical signal in the core region by adding germanium dopant to the silica glass of the core, thereby increasing the refractive index of the core with respect to the surround cladding layer. The germanium dopant, however, has been found to introduce optical loss to the propagating signal (the germanium increasing Rayleigh scattering of the light within the core). To overcome this problem, some optical fibers have been constructed with a pure silica core (i.e., a “Ge-free” core), thus minimizing losses attributed to scattering and the like. When using a pure silica core, a specialized cladding material is required that will function to confine the propagating optical mode to the core region by decreasing the refractive index of the cladding with respect to the core (referred to in the art as “down doping” of the cladding). Fluorine is one dopant that has been useful for this purpose, where the inclusion of fluorine dopant in the silica glass forming the cladding layer will decrease the refractive index of the cladding relative to the core, providing optical signal confinement within the core.
While useful in providing a low loss fiber structure with a Ge-free core, the arrangement of a pure silica core surrounded by an F-doped cladding has exhibited problems related to its fabrication. Since F-doped silica has a lower viscosity than pure silica, the act of heating an optical preform and then drawing the preform down into an optical fiber creates a situation where the more rigid core will support a majority of the draw tension, resulting in significant index reduction and high residual stress in the core region. This mechanical stress, in turn, creates glass defects that behave as scattering sites and thus increase optical attenuation.
Presently, the best solution to this residual stress problem is to reduce the draw tension by increasing the draw temperature and/or decreasing the rate at which the fiber is drawn-increasing manufacturing costs as a result.
U.S. Pat. No. 6,917,740 issued to H. D. Boek et al. on Jul. 12, 2005 addresses the viscosity mismatch problem present in a Ge-free optical fiber, and proposes the use of a core region that is co-doped with both chlorine and fluorine. The inclusion of chlorine in the core thus improves viscosity matching to the cladding and reduces the residual stress that would otherwise be present in the drawn fiber. However, the inclusion of these dopants in the core region allows for the optical power to spread into the cladding (since the refractive index difference is somewhat lessened), thus reducing the optical power present within the core region and increasing the optical power in the cladding region, resulting in a higher fiber attenuation. Additionally, the smaller index difference between the core and cladding regions further increases the bending losses found in the fiber.
Various other prior art techniques have been proposed to reduce the viscosity mismatch in optical fibers. See, for example, an article entitled “Design of Viscosity-Matched Optical Fibers” by M. Tateda et al., appearing in IEEE Photonics Technology Letters, Vol. 4, No. 9, September 1992 at pp. 1023 et seq.
A technique for addressing the residual tension present in a fiber drawn from a viscosity mismatched preform is described in U.S. Pat. No. 7,593,612 issued to I. Shimotakahara on Sep. 22, 2009. Here, a tension-absorbing cladding layer is included as an outer jacket over a conventional cladding layer, where the tension absorbing outer jacket is formed of a material that exhibits a refractive index similar to the core (such as, for example, a pure silica tension absorbing layer). During draw, the mechanical stress forces present between the mismatched core and cladding will be transferred away from the core and be absorbed through the interaction between the outer jacket and the cladding. While capable of mitigating the degree of draw-induced stress, the outer jacket is itself formed of a rigid material that necessitates a higher draw temperature that will increase the glass fictive temperature, resulting in higher attenuation. Furthermore, the addition of the tension-absorbing layer increases processing complexities. The higher index of the tension-absorbing layer relative to that of the cladding layer can also lead to power leakage from the core to the tension-absorbing layer, especially in a tightly-bent fiber; and that results in a high bending loss.